1. Field of the Invention
The present relates generally to bus driver circuit. More specifically, the invention relates to a bus driver circuit permitting adjustment of transition period of rising up and falling down of a transmitting data output and whereby realizing high speed transmission of the transmitting data.
2. Description of the Related Art
Conventional high speed bus driver for transmission of a transmitting data has different through rate (transition period in rising up of the transmitting data output) for efficiently performing high speed signal transmission depending upon the kind and nature of the data waveform to be transmitted, signal propagation speed and installation condition of other associated boards.
Accordingly, in order to perform signal transmission efficiency, it is necessary to select optimal through rate depending upon the kind and nature of the data waveform to be transmitted, signal propagation speed and installation condition of other associated boards.
However, the through rate of this type of bus driver is significantly depends on the performance of a transistor included therein. Therefore, the conventional bus driver circuit has no function for adjusting the through rate or has simple adjustment function discussed below.
One example of the conventional bus driver circuit which permits adjustment of the through rate, has been disclosed in Laid-Open application No. 2-122725. In this publication, there is a disclosure for the bus driver circuit, in which the through rate is adjusted by a control terminal and N-channel transistor and P-channel transistor In concrete, when a voltage of high potential (H) is applied to the control terminal, the N-channel transistor and the P-channel transistor are turned ON. By this, the N-channel of an inverter is strengthened so that the gate of the P-channel transistor may turn into low potential (L) at high speed. As set forth above, in this prior art, the through rate of the output buffer is adjusted.
Thus, the conventional bus driver circuit does not have the through rate adjusting function or has only a simple adjusting function. Therefore, it is not possible to efficiently perform high speed transmission of the data waveform at an optimal through rate depending upon the kind and nature of the data waveform to be transmitted, signal propagation speed and installation condition of other associated boards. Also, the foregoing conventional bus driver circuit having the through rate adjusting function, adjustment of the through rate is permitted between two levels. Therefore, in order to obtain optimal through rate, the design has to be differentiated per the system to apply.
Therefore, it has been desired a bus driver circuit which permits multi-level adjustment of the through rates.
Next, discussion will be given for the prior art in a signal output circuit for outputting a signal to be input to the bus driver circuit.
At first, in advance of discussion for the prior art, the general construction of a bus transmission path will be discussed with reference to the drawing, particularly to FIG. 12. As shown in FIG. 12, a plurality of bus transmission paths 303 are provided on a mother board 300. Also, on the mother board 300, a plurality of connectors 301 are mounted, To the connectors 301, substrates 302 are connected. Thus, the internal circuit of the substrates 302 are connected to the bus transmission paths. Each of the substrates 302 receives and transmits signals via the bus transmission paths.
A characteristic impedance of the bus transmission paths may be varied depending upon the various factors. A major factor to cause variation in the number of substrates to be installed to the connector 301 is yield in fabrication. Amongst, discussion will be given for variation of the impedance with reference to the drawing, particularly to FIG. 12,
In the construction of the bus transmission path as illustrated in FIG. 12, the number of substrate 302 to be connected to the bus transmission path 303 is not constant. For example, in FIG. 13, only substrates 302 at both ends are connected to the connector 301. Therefore, number of the substrate is two. On the other hand, in FIG. 14, the number of the substrate to be installed becomes seven.
By variation of the number of the installed substrates, the characteristic impedance of the bus transmission path 303 is varied. In order to show this, the approximated characteristic impedances of respective constructions of FIGS. 13 and 14 are calculated.
In the bus transmission path of the construction shown in FIG. 12, it is assumed that the distance between the connectors 301 is 1 inch (2.54 cm), the characteristic impedance of the bus transmission path in the case where no substrate is installed is Z0:0.75 .OMEGA. and propagation delay period is t=7 ns/m. At this time, an inductance component L0 and capacitance component C0 are approximately calculated as 13.5 nH/inch and 2.36 Pf/inch.
Here, assuming that 25 pF of the capacitance component is increased per each substrate 302, the characteristic impedance Z1 of the bus transmission path 303 in the construction of FIG. 13 can be calculated as 32.2 .OMEGA.. On the other hand, the characteristic impedance Z2 of the bus transmission path 303 in the construction of FIG. 14 becomes 20.5 .OMEGA.. Namely. when the construction of FIG. 14 is constructed by adding five substrates 302 for the construction of FIG. 13, the characteristic impedance is lowered in the extent of 14.7 .OMEGA..
In the discussion given hereabove, variation of the characteristic impedance is theoretically calculated with employing approximation for simplification. However, in practice, it is difficult to predict variation of the characteristic impedance in advance of actual installation of the substrate. For example, the characteristic impedance may be variable not only depending upon the number of substrate to be installed in the bus transmission path but also depending upon the position of installation of the substrate. Thus, the characteristic impedance of the bus transmission path is variable depending upon various factors. Then, associating with variation of the characteristic impedance, the signal waveform to be propagated on the bus transmission path may be differentiated.
For example, in case of a pulse wave, when the characteristic impedance of the bus transmission path is excessively large, the rising up period of the pulse becomes long. On the other hand, when the characteristic impedance is too small, ringing may be caused. Ringing is a transitional vibration of the waveform to be caused by abrupt rising up of the pulse and can be a cause of malfunction.
Beside, in order to propagate signal at high speed, signal has to be maintained at constant waveform. Therefore, the characteristic impedance of the bus transmission path has to be corrected to be a given constant value.
One example of the conventional signal output circuit having an adjusting function for the characteristic impedance of the bus transmission path is shown in FIG. 15. With reference to FIG. 15, between the output portion 311 for outputting the signal and the bus transmission path 333, a resistor 351 is connected. In this signal output circuit, by varying the resistance of the resistor 351, the characteristic impedance is adjusted to shape the signal waveform into a desired shape. The resistance of the resistor 351 is determined depending upon the characteristic impedance of the bus transmission path 333.
However, in the above-mentioned conventional signal output circuit, when the characteristic impedance is varied by modification of the installation condition of the substrates in the bus transmission path, it becomes necessary to exchange the resistor per se in order to vary the resistance value to make handling cumbersome.
Also, as set forth above, it is difficult to even theoretically predict the characteristic impedance of the bus transmission path. Therefore, the resistance value of the resistor to be employed in the signal output circuit has to be obtained through experiments. At this time, it is required to repeat cumbersome operation to exchange the resistors.